In construction applications, mortars may be prepared with cement, sand, and organic polymer. To reduce shipping costs, the polymer can be shipped and added in dry form as a redispersible polymer powder. Redispersible polymer powders improve the adhesion and flexibility of cement based tile adhesives. The powdered form of the polymer is generally produced by spray drying a liquid polymer composition to obtain a free flowing powder. To perform its function in the application formulation to which it is added, such as cementicious systems, it is desired that in the application formulation the polymer powder is easily redispersible. Also, in preparing a redispersible polymer powder (RDP) from a latex or polymer dispersion by spray drying, a low viscosity polymer dispersion is desired to enable the use of higher solids content compositions for easier spray drying and lower pressure equipment for more efficient production of RDPs without loss of redispersibility.
The properties of adhesion, wear resistance, scratch resistance and bend resistance of hydraulically setting compositions are generally improved by adding dispersion powders. Polyvinyl acetate dispersion powders as an additive in flowable compositions are generally in dry mortar from when they are delivered to the building site, where they are simply mixed with water and spread on the floor. The materials flow out to give a smooth surface which serves directly as the wear layer or serves as substrate for further coatings. However, this usage exhibits problems, particularly when relatively thick layers are applied, such as the formation of uneven areas, such as craters or pinholes, which form on the surface. The surface does not become as smooth as the purchaser desires, and requires further work. To avoid uneven areas of this type, additives, such as fully hydrolyzed copolymers made from 1-alkylvinyl esters and from vinyl esters have been employed to eliminate these problems.
U.S. Pat. No. 6,632,861 to Weitzel et al discloses that the use of powders based on protective-colloid-stabilized vinylaromatic-1,3-diene polymers enables the use of expensive, difficult to prepare, fully hydrolyzed copolymers made from 1-alkylvinyl esters and from vinyl esters to be dispensed with, while retaining equivalent surface qualities and at the same time obtaining good wear resistance, scratch resistance and adhesion. The Weitzel et al water-redispersible, protective-colloid-stabilized dispersion powder compositions for use in self-leveling screeds or trowel-applied flooring compositions, include: a) a base polymer selected from the group consisting of the vinylaromatic-1,3-diene polymers, b) from 2 to 25% by weight, based on the base polymer, of one or more protective colloids, c) from 3 to 30% by weight, based on the total weight of polymeric constituents, of fine antiblocking agent, and d) from 0.1 to 10% by weight, based on the base polymer, of other additives.
It has been found that the use of carboxylated styrene-butadiene redispersible powders as an additive in self-leveling flooring compounds or compositions generally improves the abrasion resistance, surface appearance, and color compared to systems modified with powders based in other chemistries like vinylacetate/ethylene copolymers. However, improvement in overall bonding strength without adversely affecting rheology in a cement-based formulation such as a self-leveling flooring composition, and a cement based tile adhesive formulation, and without adversely affecting redispersibility of the carboxylated styrene butadiene redispersible polymer powder would be highly desirable.
Mortars formulated with the carboxylated styrene butadiene redispersible polymer powder compositions of the present invention exhibit an unexpectedly superior increase in bonding strength after water immersion without adversely affecting rheology of the cementicious mortar or redispersibility of the polymer powder relative to vinylacetate/ethylene copolymer RDPs and other SB RDPs, which is advantageous for workability or ease of troweling during application. Accordingly, the present invention solves the problems of inadequate abrasion resistance, surface appearance, control of cement setting and off-colors while achieving unexpectedly superior overall bonding strength for cement compositions prepared from dry mix formulations.